2.8 Shortcut to turn on/off "Emulate 3 Button Mouse"..?
Blender's keymap requires an operator to execute when the shortcut is pressed. Currently there is no operator to toggle the Emulate 3 Button Mouse option from the preferences, but this can be solved by writing a little add-on.

The following add-on creates an operator that allows to toggle the preference option and can therefore be added as a shortcut into your keymap. Copy &amp; paste the script into a file and save it with the .py file extension. 

bl_info = {
    "name": "Toggle 'Emulate 3 Button Mouse'",
    "author": "Robert Guetzkow",
    "version": (1, 0),
    "blender": (2, 80, 0),
    "location": "Edit &gt; Operator Search",
    "description": "Operator for toggeling the 'Emulate 3 Button Mouse' option.",
    "warning": "",
    "wiki_url": "",
    "category": "Preferences"}

import bpy


class PREFERENCES_OT_toggle_emulate_3_button_mouse(bpy.types.Operator):
    bl_idname = "preferences.toggle_emulate_3_button_mouse"
    bl_label = "Toggle 'Emulate 3 Button Mouse'"

    def execute(self, context):
        context.preferences.inputs.use_mouse_emulate_3_button = not context.preferences.inputs.use_mouse_emulate_3_button
        return {"FINISHED"}


classes = (PREFERENCES_OT_toggle_emulate_3_button_mouse,)


def register():
    for cls in classes:
        bpy.utils.register_class(cls)


def unregister():
    for cls in classes:
        bpy.utils.unregister_class(cls)


if __name__ == "__main__":
    register()


You can install the add-on in Edit > Preferences > Add-ons. 



Once it's installed you will have to enable it. You can find it in the Preferences category.



Then switch to the Keymap in the preferences and add a shortcut referencing the operator from the add-on. You'll have to decide what key binding you'd like to use.